Efforts are being made to evolve cellular mobile communication systems from UMTS (Universal Mobile Telecommunication System) into LTE (Long Term Evolution). With LTE, OFDM (Orthogonal Frequency Division Multiplexing) and SC-FDMA (Single Carrier-Frequency Division Multiple Access) are respectively employed as downlink and uplink wireless access techniques, and a high-speed wireless packet communication at a downlink peak transmission rate of 100 Mb/s or faster, and at an uplink peak transmission rate of 50 Mb/s or faster, is enabled. 3GPP (3rd Generation Partnership Project), an international standardization organization, has launched a study of an LTE-based mobile communication system LTE-A (LTE-Advanced) to realize a higher-speed communication. With LTE-A, a downlink peak transmission rate of 1 Gb/s and an uplink peak transmission rate of 500 Mb/s are pursued as a goal, and diverse new techniques for wireless access schemes, network architectures, and the like are currently being studied (Documents 1-6). In the meantime, since LTE-A is a scheme based on LTE, efforts are being made to maintain backward compatibility.
As one method of performing a high-speed data communication, a method of introducing relay stations (RNs: Relay Nodes) is currently being studied, as illustrated in FIG. 1, in order to support a communication between a base station and a mobile station (Document 2). The relay station is installed to support high-speed data communication by relaying between a base station (Doner eNB or eNB) and a mobile station (UE: User Equipment). As illustrated in FIG. 2, a link between the mobile station UE and the relay station RN, and a link between the base station (eNB) and the relay station (RN) are referred to as Uu and Un, respectively. In the following description, Uu and Un are sometimes referred to as an access link and a backhaul, respectively.
Various types of schemes are considered as forms of the relay station. A repeater scheme, a decode-and-forward scheme, and L2 and L3 schemes are the main ones being studied. Here, a relay station of the repeater scheme only has a function of amplifying a wireless signal (data signal and noise). A relay station of the decode-and-forward scheme has a function of amplifying only a data signal within a wireless signal. A relay station of the L2 scheme has an L2 function such as a MAC layer. A relay station of the L3 scheme has an L3 function, such as an RRC layer or the like, and operates similarly to a base station. The relay station of the L3 scheme is called “Type1 RN” in LTE-A.
A method of deploying a relay station in a cell is also under review. For example, a deployment method of installing a relay station at an edge of a cell in order to increase a throughput at the edge of the cell, and a deployment method of installing a relay station in a range (dead spot) locally unreachable by radio waves from a base station within a cell are mainly being studied.
For a relay (inband relaying) that shares the same frequency band between a base station and a relay station and between the relay station and a mobile station when data is transmitted/received between the base station and the mobile station via the relay station of the L3 scheme (Type1 RN), it is preferable not to cause self-interference in the relay station. Self-interference (also called loop interference) is interference such that, for example, when a relay station transmits downlink data from the relay station to a mobile station at the same time that the relay station receives downlink data addressed to the relay station from a base station, the transmitted data gets into a receiver module of the relay station and interferes with the data received from the base station. Self-interference may similarly occur in the case of uplink data. When self-interference occurs, a relay station cannot properly receive data.
To overcome this self-interference problem, a study of LTE-A is currently underway based on the following policies (Document 2).
(A) Downlink: A relay station does not transmit data to a mobile station in a downlink backhaul (DL backhaul) that is a subframe with which data is received from an upper-level base station.
(B) Uplink: A relay station does not receive data from a mobile station in an uplink backhaul (UL backhaul) that is a subframe with which data is transmitted to an upper-level base station.
Based on the above described policy (A), when a downlink backhaul is configured between a relay station and a base station, a subframe between the relay station and a mobile station is configured as an MBSFN (Multicast/Broadcast over Single Frequency Network) subframe as illustrated in FIG. 3. The reason is as follows. A mobile station conforming to LTE does not receive unicast data with an MBSFN subframe. Accordingly, the mobile station UE does not receive some of reference signals, and does not need to unnecessarily measure the reference signals, which is advantageous. More specifically, in a downlink backhaul, the relay station can transmit a PDCCH (Physical Downlink Control Channel), a PHICH (Physical Hybrid ARQ Indicator Channel), and a PCFICH (Physical Control Format Indicator Channel) as control signals to the mobile station, but cannot transmit a PDSCH. The reference signal is arranged in the first half (CTRL section in FIG. 3) of the MBSFN subframe in order to receive the control signal. However, the reference signal is not arranged in the latter half of the MBSFN subframe.
Based on the above described policy (B), the relay station performs a control so as not to give an uplink data transmission grant (UL grant) to the mobile station 4 subframes (4 ms) before an uplink backhaul. This control is performed to avoid a situation where the mobile station transmits data to the relay station in the uplink backhaul if the relay station gives the uplink data transmission grant to the mobile station 4 ms before the uplink backhaul.
The relay station also performs a control so as not to make a downlink data transmission to the mobile station 4 subframes (4 ms) before the uplink backhaul. This control is performed to avoid the following situation. A HARQ (Hybrid Automatic Repeat reQuest) of LTE is specified such that a destination station returns an ACK/NACK signal 4 ms (time period corresponding to 4 subframes) after a data transmission made by a source station. Accordingly, if the relay station transmits downlink data to the mobile station during 4 ms of the uplink backhaul, the mobile station returns the ACK/NACK signal to the relay station in the uplink backhaul.
Note that, in the uplink backhaul, a PUCCH (Physical Uplink Control Channel) and a PUSCH (Physical Uplink Shared Channel) that are control signals to the relay station can be transmitted, but a PUCCH and a PUSCH that are control signals from the mobile station cannot be transmitted.
There are some documents which relate to the present application.    Document 1: 3GPP TR 36.913 V8.0.1 (2009-03), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Requirements for further advancements for Evolved Universal Terrestrial Radio Access (E-UTRA) (LTE-Advanced) (Release 8)    Document 2: 3GPP TR 36.912 V9.0.0 (2009-09), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Feasibility study for Further Advancements for E-UTRA (LTE-Advanced) (Release 9)
Document 3: 3GPP TS 36.133 V8.8.0 (2009-12), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Requirements for support of radio resource management (Release 9) Document 4: 3GPP TR 36.806 V2.0.0 (2010-02), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Relay architectures for E-UTRA (LTE-Advanced) (Release 9)    Document 4: 3GPP TR 36.806 V2.0.0 (2010-02), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Relay architectures for E-UTRA (LTE-Advanced) (Release 9)    Document 5: GPP TR 36.423 V9.0.0 (2009-09), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 Application protocol (X2AP) (Release 9)    Document 6: 3GPP TR 36.413 V9.0.0 (2009-09), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application protocol (S1AP) (Release 9)
In a case where a relay station is fixedly arranged, a backhaul configured between the relay station and a base station that covers a cell where the relay station is arranged is considered to be almost fixed and not to vary with time. However, a backhaul of a moving relay station (hereinafter referred to as a mobile relay station as needed) can be changed with a handover of the relay station (see FIG. 4) according to a connected base station. Accordingly, when a handover occurs, the mobile relay station notifies a mobile station under the mobile relay station of a new MBSFN subframe as a downlink backhaul used after the handover. The new MBSFN subframe is implemented by updating broadcast information (a BCCH (Broadcast Control Channel)).
However, time is needed to update the broadcast information. This point is described with reference to FIG. 5. FIG. 5 is an explanatory view of a relationship between an update of broadcast information and an update of a backhaul.
To update the broadcast information, the mobile relay station initially makes an advance notification of updating the broadcast information to the mobile station by transmitting a Paging message or broadcast information including a Value Tag. Here, the broadcast information is updated by using a duration “BCCH modification period” as a reference. Namely, the contents of the broadcast information are reflected in the next BCCH modification period after the advance notification is given. With reference to FIG. 5(a), if the current BCCH modification period is assumed to be a BCCH modification period (n), and if the advance notification is given within a duration of this BCCH modification period (n), the broadcast information is applied in the next duration BCCH modification period (n+1). A boundary between adjacent BCCH modification periods is referred to as “BCCH modification boundaries”. Namely, after the advance notification is given, new broadcast information is applied when the next BCCH modification boundary is passed. According to LTE, if a length of the duration of the BCCH modification period is assumed to be m, m={640 ms, 1280 ms, 2560 ms, . . . , 40960 ms} is specified, and BCCH modification boundaries=SFN mod m (SFN is a frame number (System Frame Number) are specified.
As described above, there is a time lag between the time when an advance notification is given and the time when new broadcast information is applied based on the advance notification. Therefore, a new backhaul based on the broadcast information cannot be configured between these times. With reference to FIG. 5(b), a handover (HO) of the mobile relay station is detected at a certain timing during the BCCH modification period (n), and an advance notification is given to a mobile station under the mobile relay station. New broadcast information based on the advance notification is applied in and after the BCCH modification period (n+1). Accordingly, a backhaul cannot be configured between the mobile relay station and a target base station of the handover during the first period from the time when the handover is detected and the advance notification is given up to the start time (namely, the next BCCH modification boundaries) of the BCCH modification period (n+1).
In the meantime, measures to continuously configure a backhaul used before the handover are considered to be taken during the first period. However, there is a situation in which it is difficult to take these measures. This point is described with reference to FIG. 6 and FIG. 7. FIG. 6 and FIG. 7 are explanatory views of the situation in which it is difficult to continuously configure the backhaul used before the handover after the handover is performed. In these figures, (a) schematically illustrates a state before the handover is complete, and (b) schematically illustrates a state after the handover is complete.
The example illustrated in FIG. 6(a) represents a case where a subframe #1 (MBSFN=1) from among 10 subframes #0 to #9 within one frame is configured as an MBSFN subframe of a mobile relay station RN3 before the handover occurs. At this time, assume a situation where the subframe #1 is used as an MBSFN subframe (MBSFN=1) for a mobile station and a downlink backhaul is configured in a subframe #3 (DL_BH=3) for a relay station RN1 in a target base station eNB of the handover. In such a situation, the mobile relay station cannot configure the subframe #1 as a downlink backhaul after the handover is performed.
The reason is as follows. The subframe #1 that is used as the MBSFN subframe for the mobile station under the target base station eNB is transmitted to the mobile station in the signal format dedicated to MBSFN as illustrated in FIG. 3. In contrast, even if the downlink backhaul of the mobile relay station RN3 after the handover is attempted to be configured in the subframe #1, a signal is transmitted in the signal format dedicated to unicast data in the backhaul. Therefore, both of these signals are inconsistent with each other in the subframe #1.
Accordingly, in the example illustrated in FIG. 6(a), it is preferable that the mobile relay station RN3 changes the downlink backhaul to a subframe other than the subframe #1; for example, the subframe #3 (FIG. 6(b)).
The example illustrated in FIG. 7(a) represents the case where the subframes #1 and #3 (MBSFN=1/3) among the 10 subframes #0 to #9 within one frame are configured as the MBSFN subframes of the mobile relay station RN3. At this time, assume a situation where a downlink backhaul is respectively configured in the subframes #1 and #3 (DL_BH=1/3) for relay stations RN1 and RN2 in the target base station eNB of the handover. In such a situation, the downlink backhaul configured before and after the handover is the same. In this case, it may be difficult to configure the same backhaul as that of the relay stations RN1 and RN2 in the mobile relay station RN3 from the viewpoint of a traffic load and QoS. In such a case, it is preferable that the mobile relay station RN3 changes the downlink backhaul to subframes other than the subframes #1 and #3; for example, the subframes #6 and #8 (FIG. 7(b)).